Vapor deposition process

ABSTRACT

A vacuum vapor deposition process is described for depositing a coating of an iron-base alloy on a substrate. The quality of the deposited coating is improved by maintaining the substrate above a minimum temperature of 1,600* F., in the case of alloys with substantially soluble constituents, and of 1,700* F., in the case of alloys having at least partially insoluble constituents.

Smith ..164/46 X O United States Patent 1151 3,652,325 Hunt 1451 Mar. 28, 1972 [54] VAPOR DEPOSITION PROCESS 3,290,126 12/1966 Monson ..117/107.2 x 3,312,546 4/1967 Mayer et al..... ...1l7/l07.2 X [72] Charles D A Cahf' 3,477,831 11/1969 Talboom el al. ..117/107.2 x [73] Assig nee: Air Reduction Company, Incorporated Primary Examiner-Alfred L. Leavitt [22] led: 1968 Assistant Examiner-W. F. Boll 21 Appl 790 500 AttorneyFitch, Even, Tabln & Luedeka [57] ABSTRACT [52] U.S.Cl ..1l7/l07,1l7/107.1, 164/46 5 1 1 13/02 A vacuum vapor deposition process is described for depositm g 58 Field of Search ..117 107 107.1 1072- 164/46 a mating imn'base a1lY a substrate- The qualiy the deposited coating is improved by maintaining the substrate f 4 above a minimum temperature of l,600 F., in the case of al- [56] Re erences cued loys with substantially soluble constituents, and of l,700 F UNITED STATES PATENTS in the case of alloys having at least partially insoluble constituents. 3,096,160 7/1963 Puyear.... ..1 17/1072 X 3,270,381 9/1966 3 Claims, 6 Drawing Figures FlG.lu

FIGZb mvsmron CHARLES d'A HUNT VAPOR DEPOSITION PROCESS This invention relates to vacuum vapor deposition and, more particularly, to vacuum vapor deposition of iron-base alloys in a manner which provides a coating of improved quality.

The technique of evaporating and condensing various materials on a substrate in a vacuum is known in the art as vacuum vapor deposition. Vacuum vapor deposition may be utilized to produce coatings on substrates for various purposes. Iron-base alloys may be utilized in coatings produced by vacuum vapor deposition for a number of reasons, one significant reason being to provide resistance against corrosion. For example, turbine blades subject to high temperature operation may be comprised of nickel or a material having similar high strength properties at elevated temperatures. Nickel and similar materials, however, are sometimes subject to excessive corrosion, and a coating of a corrosion resistant iron-base alloy may be provided for protection.

Coatings that are designed to protect a substrate against corrosion should be relatively free of defects, such as microscopic cracks and similar discontinuities, or corrosion will penetrate the coating and attack the substrate, possibly leading to failure. In producing a coating by means of vacuum vapor deposition, defects in the coating may be caused by three factors:

1. Flaws and other discontinuities previously existing in the surface of the substrate;

2v Particles, such as droplets of splattered liquid metal or flakes of condensate, landing on the substrate during coating; and

3. Long continuous grain boundaries in the coating extending in a straight line from the outer surface of the coating to the substrate surface.

Flaws in the surface of the substrate may consist of any part of the substrate surface which is not substantially parallel with the remainder of the substrate surface. A step, indentation or scratch in the substrate surface may constitute such a surface flaw that will be continued as a discontinuity all the way to the surface of the deposited coating. In some cases, a flaw on the substrate surface may even be magnified in the formation of the discontinuity in the deposited coating. Microscopic cracks in the coating often develop between grains adjacent such a discontinuity. This often leads to very rapid attack of the substrate if subjected to a corrosive environment.

In addition to substrate surface flaws, particles landing on the part during the coating process may also produce discontinuities in the coating. Such particles may consist of splattered droplets from the molten liquid which is being evaporated, or may comprise flakes of condensate which fall off of cooled parts within the vacuum vapor deposition furnace and land on the part. Such a particle may not create a discontinuity that extends all the way from the surface of the coating to the substrate. Nevertheless, the discontinuity which is produced, may cause development of microscopic cracks in the coating as in the case of substrate surface flaws. This decrease in the integrity of the coating makes the coating and the substrate it covers more susceptible to attack by corrosion.

Grain boundaries of evaporated and condensed material generally appear to be substantially weaker mechanically and more susceptible to corrosion than are grain boundaries found in metal which is frozen directly from a molten state. Frequently, in vacuum vapor deposited coatings, the grains appear to grow almost independently and the grain boundaries have little or negligible strength. If, in addition to having generally weaker grain boundaries, the coating is of a very columnar grain structure in which the grain boundaries extend continuously in a straight line from the outer surface of the coating to the substrate surface, the substrate is susceptible to corrosive attack by preferential corrosion along the grain boundaries.

It is known that the physical characteristics of a vacuum vapor deposited coating may be influenced by the temperature of the surface of the substrate upon which the coating is deposited. A process in which the substrate surface is maintained at an elevated temperature during deposition is described in U.S. Pat. No. 3,270,381 wherein foil of improved ductility is produced. The minimum temperature above which the substrate surface is maintained in the aforesaid patent is of at least about 25 percent of the absolute melting point of the coating material. As pointed out in that patent, the reason for the transformation of the deposited coating from a brittle condition to a ductile condition at substrate temperatures above 25 percent of the absolute melting point of the coating material is not fully understood. The deposited coating under such conditions has a columnar grain structure when deposited on a substrate maintained at a temperature either slightly above or slightly below 25 percent of the absolute melting point of the coating material and it is difiicult to detect any difference in the deposited coatings under magnification.

It is the principal object of this invention to provide an improved process for coating a substrate with an iron-base alloy.

Another object of the invention is to provide a process of vacuum vapor deposition of iron-base alloys wherein substrate surface flaws and particle inclusions in the coating do not result in a substantial reduction of the integrity of the coating.

It is another object of the invention to provide a coated substrate product having a vacuum vapor deposited coating of improved quality.

Other objects of the invention will become apparent to those skilled in the art from the following description taken in connection with the accompanying illustrations wherein:

FIG. 1 is a series of four cross sectional photomicrographs, enlarged times, of coatings of ASIE Type 430 stainless steel on a mild steel substrate at various temperatures;

FIG. 2 is a series of two cross sectional photomicrographs of a coated substrate product in which the substrate comprises mild steel and the coating comprises 25 percent chromium, 6 percent aluminum, 0.4 percent yttrium, balance essentially iron. FIG. 2a is enlarged 100 times and FIG. 2b is enlarged 400 times.

Very generally, the method of the invention comprises vaporizing the iron-base alloy in a vacuum and condensing at least a portion of the vaporized alloy on the substrate. The surface of the substrate upon which the alloy is condensed is maintained at a temperature which is above a minimum temperature during condensation of the metal or alloy. 1n the case of alloys having substantially soluble constituents at the deposition temperature, the minimum temperature is about 1,600" P. In the case of alloys having at least partially insoluble constituents at the deposition temperature, the minimum temperature is about 1,700 F.

The method of the invention may be practiced in a vacuumtight enclosure evacuated to a low pressure, for example less than 1 Torr. The substrate upon which a coating is to be deposited is disposed in the enclosure. The substrate may be a flexible web or sheet of material passed from a supply roller to a take-up roller, or may be a discrete object suitably supported and, if desired, rotated within the vapor flow. Vacuum valves may be provided in the walls of the enclosure through which the substrate may be passed into and out of the enclosure.

The iron-base alloy which is to be deposited on the substrate may be contained within a crucible. The crucible may be comprised of refractory material, however, it is preferred that the crucible be water cooled copper or stainless steel and that heating of the material therein be accomplished by means of electron beams. In this manner, high purity and close control of coating composition may be achieved. Apparatus for vapor deposition which is constructed generally in accordance with the foregoing description is shown and described in U.S. Pat. No. 3,183,563, assigned to the assignee of the present invention. Such apparatus may be utilized for carrying out the method of the present invention in order to produce the product of the invention.

In practicing the method of the invention, the substrate is disposed in a vacuum chamber and the iron-base alloy to be deposited thereon is vaporized. At least a portion of the evaporated iron-base alloy is allowed to condense on the substrate. Prior to and during condensation, the surface of the substrate is maintained above a minimum temperature. The temperature is selected to be sufficiently high as to impart enough mobility to the atoms of the deposit to overcome the barriers of potential energy present in the crystalline deposit structure so that the condensing atoms have a tendency to seek a smooth surface. This conforms with laws of physics which indicate that atoms, when mobilized, tend to seek the condition of least potential energy i.e. greater entropy).

Heating of the substrate may be accomplished by a heater of the resistance or inductance type or, if desired, by an electron beam gun which directs a beam of electrons at the uncoated side of the substrate or at a radiant heater element placed nearby. The arrangement of the heaters should be such as to avoid heating the substrate to a temperature at or exceeding the re-emission temperature of the evaporant, since under the latter condition no condensation will occur.

If heating is provided to impart sufficient mobility to the atoms of the deposit, not only will the deposit tend to cover up surface flaws in the substrate, but particles landing on the coating during the deposition process are also often masked.

Moreover, the tendency for microscopic cracks to develop as a result of discontinuities in the coating is reduced. Furthermore, improved strength in grain boundaries appears to occur throughout the deposit when produced in accordance with the invention, thereby rendering the deposit less susceptible to selective corrosion along grain boundaries.

For some iron-base alloys, condensation at substrate temperatures that are elevated in accordance with the invention will not only provide superior grain boundary strength and integrity, and a superior ability to cover up surface defects and particles, but will also produce a random distribution of the grains in the coating. For many materials which normally tend to have a columnar distribution of grains when vapor deposited, a transition temperature exists above which the distribution of the grains is generally random. Such transition temperatures generally exceed about 50 percent of the absolute melting temperature of the deposit for iron-base alloys having isotropic crystalline structures at their condensation temperatures and which, after condensation, form substantially homogeneous solid solutions. Such iron-base alloys include iron-nickel and iron-chromium alloys. This is typically true also for pure iron. Deposits. in which the grains are randomly distributed provide a significant advantage from the standpoint of resisting corrosion. This is because there are no grain boundaries which extend in a generally straight line from the surface of the substrate to the surface of the coating and along which preferential grain boundary corrosion may occur.

It has been found that for improved but not necessarily random-grain coatings of those iron-base alloys comprised of constituents which are substantially soluble at the deposition temperature, the minimum temperature at which the surface of the substrate should be maintained is about l,600 F. for satisfactory results. On the other hand, where insoluble phases are formed at the deposition temperature, such as in the case of yttrium and the rare earth metals, it is typically necessary to exceed about 1,700 F. for satisfactory results. In either circumstance, vacuum vapor deposition carried out in accordance with the invention provides a coating in which microscopic cracks resulting from discontinuities propagated from surface defects and particle inclusions are minimized. Moreover, the integrity of the grain boundaries in coatings produced in accordance with the invention appears superior to coatings obtainable with heretofore known methods. This is true even though grain distribution is not random, although the latter situation may provide additional corrosion resistance due to the avoidance of long grain boundaries extending from the surface of the deposit to the substrate.

It is generally desirable that the coating not comprise a single crystal, since it may be more susceptible to fracture and corrosion. Accordingly, the temperature is maintained below that at which epitaxial growth will occur, the latter depending upon conditions of purity and temperature.

Iron-base alloys having soluble phases at deposition temperatures and for which superior quality deposits have been attained in accordance with the invention include iron-nickel, iron-copper and iron-chromium alloys. Referring now to FIG. 1, an example of a soluble phase type of iron-base alloy is shown. Cross sectional photomicrographs'of deposited layers of ASIE 430 stainless steel deposited on mild steel at various substrate temperatures are illustrated. Type 430 stainless steel comprises about 26 percent chrome, the balance essentially iron and represents a situation where the constituent elements are substantially soluble at the deposition temperature. In FIG. 1a, the temperature was l,000 F. A large scratch in the surface of the substrate (the large rounded depression with ridges at each end) existed and it may be seen that the scratch is reproduced in detail on the surface of the deposit and that, moreover, two long microscopic cracks are propagated from the edges of the crack to near the surface of the deposit. Corrosion along such cracks is likely to occur.

FIG. 1b shows a similar sample deposited at 1,200 F. A scratch was also placed in the surface of the substrate and it may be seen that a microscopic crack was propagated all the way to the surface of the deposit from the surface of the sub- .strate. Corrosion along this crack can attack the substrate readily.

It may also be observed that in the case of l,200 F as well as the case of 1,000 F. in FIG. 1a, the grain distribution is substantially columnar in the deposit.

FIG. 10 illustrates the same type of coating deposited at a temperature of 1,600 F. It may be seen that the scratch existing on the surface of the substrate did not result in microscopic cracks being propagated to the surface of the deposit. Moreover, the scratch in the substrate was masked somewhat by thedeposited coating. The distribution of the grains in the coating tends toward being random.

In FIG. 1d, the temperature of the substrate was maintained at about 1,800 F. Although the scratch in the surface of the substrate was not masked appreciably, no microscopic cracks were propagated between the substrate and the surface of the coating. Moreover, a splatter droplet impinging upon the coating about half-way through the deposition process (the area surrounded by the dark dots which are precipitates) was covered up well by the deposit and did not result in the propagation of any microscopic cracks. The distribution of the grains in the coating of FIG. 1d is substantially random.

Iron-base alloys having insoluble phases present at the deposition temperature and for which superior quality deposits may be attained in accordance with the invention include iron-chromium-aluminum, iron-aluminum-yttrium, iron-gadolinium, and iron-titanium alloys. Referring now to FIG. 2, a case of mutually insoluble phases being present at the deposition temperatures is illustrated. In this case, the alloy comprises 25 percent chromium, 8 percent aluminum, 0.4 percent yttrium, balance essentially iron. In FIG. 3a, the deposition temperature was l,400 F. It may be seen that a scratch (the slight depression) in the surface was not masked well. Moreover, cracks have a tendency to develop as a result of the discontinuity produced by the scratch. In FIG. 3b, the substrate temperature was about l,700 F Although the deep scratch is not masked, no microscopic cracks are present as a result of the discontinuity produced by the scratch. Moreover, although the distribution of the grains is still generally columnar, superior corrosion resistance occurs, indicating that the integrity of the grain boundaries is improved. Masking properties may be enhanced by more elevated substrate temperatures, such as about 2,000 F.

It may therefore be seen that the invention provides an improved method for vacuum evaporating and depositing ironbase alloys on a substrate. The development of microscopic cracks is substantially avoided, even in the presence of substrate surface imperfections and particle inclusions. Moreover, the integrity of the grain boundaries is superior and thereby provides superior corrosion protection for the substrate. In some cases, the grain distribution of the coating is random, enhancing the ability of the coating to withstand corrosion. The invention is applicable in depositing several successive layers of iron-base alloys of different compositions, rather than the single layer.

Various modifications of the invention will be apparent to those skilled in the art from the foregoing description and accompanying illustrations. Such modifications are intended to fall within the scope of the appended claims.

What is claimed is:

1. A process for coating a substrate with an iron-base alloy containing at least one of the elements nickel, copper, chromium, aluminum, yttrium, gadolinium and titanium, comprising: vaporizing the iron-base alloy in a vacuum, condensing at least a portion of the vaporized alloy on the substrate, and maintaining the surface of the substrate upon which the alloy is condensed above a minimum temperature during condensation of the alloy, said minimum temperature being about l,600 F. in the case of alloys having only at least one of the substantially soluble constituents nickel, copper and chromium at the deposition temperature, and being about l,700 F. in the case of alloys having at least one of the partially insoluble constituents aluminum, yttrium, gadolinium and titanium, at the deposition temperature.

2. A method according to claim 1 wherein the substrate surface is maintained at a temperature below that at which epitaxial growth will occur.

3. A method according to claim 1 wherein the coating formed is sufficiently thick to comprise, generally, at least two layers of grains. 

2. A method according to claim 1 wherein the substrate surface is maintained at a temperature below that at which epitaxial growth will occur.
 3. A method according to claim 1 wherein the coating formed is sufficientlY thick to comprise, generally, at least two layers of grains. 